1. Field of the Invention
The present application relates generally to methods and devices for selecting handover parameters in cellular networks, such as, but not limited to, a Universal Mobile Telecommunications System Radio Access Network (UTRAN).
2. Description of the Related Art
In cellular networks, handover is often a functionality that switches the user equipment (UE) from one cell to another for any of various reasons. One of the main reasons for handover is usually when another cell can provide a service with less power, in other words, less link budget. There are several parameters that generally control the handover decision making in a noisy multi-path channel. These parameters may be optimized, depending on various conditions, typically to maximize the network capacity and/or to enable auto configuration in the UTRAN. Usually, in the time division duplex (TDD) mode of the UTRAN, a Wideband Code Division Multiple Access (WCDMA) handover procedure is performed as described, for example, in the Third Generation Partnership Project (3GPP) specification TR 25.922, v.0.5.0, “Radio Resource Management Strategies”.
In many situations, the handover procedure includes of a set of parameters to be set. These parameters usually include, among others, a hysteresis value, typically in the active set update, and the length of an averaging window, usually for power measurements. These parameters often play an important role in adjusting the sensibility of handover, especially in the presence of fast channel variations and/or measurement errors. The added hysteresis normally prevents unnecessary and/or frequent handovers, also known as ping-ponging, that often disturbs quality of the connection, reduces the overall system capacity, and/or introduces unnecessary signaling load. Furthermore, measured received signal code power (RSCP) values generally are averaged over some time periods, usually in order to prevent handover that is typically due to short-term variations in the signal.
In many real networks, some delay is generally also involved in the handover execution. When, for example, a terminal device and/or user equipment (UE) according to the UTRAN terminology, triggers a handover report, which commonly means that the RSCP value of a candidate cell exceeds the RSCP value of the active cell by the hysteresis value, it typically takes some time before the handover report is delivered to the network device responsible for the handover operation, for example, a radio network controller (RNC). Furthermore, some delay is normally involved in the processing of a handover message, setting-up of a connection between the RNC and a serving base station, and/or Node B according to the UTRAN terminology, and/or allocation of radio resources for the UE in the new Node B. Further details concerning these measurements and related accuracy requirements can be gathered, for example, from the 3GPP specifications TS25.123, “Requirements for Support of Radio Resource Management (TDD)”, and TS25.225, “Physical Layer Measurements”.
Usually, the handover decision making is based on a comparison between an observed value and a predetermined threshold value, wherein the threshold value is typically chosen in a manner so as to maximize the system capacity under various conditions. However, the above handover delay commonly causes interference and switching back and forth generally causes additional interference, usually due to a prolonged connection to the old cell. Moreover, if the channel conditions are not favorable to sustain the connection in the new cell, correction of the improper handover decision is usually delayed and/or additional interference is often generated.